The prime neuropathological event distinguishing Alzheimer's disease (AD) is deposition of the 40-42 residue amyloid β-peptide (Aβ) in brain parenchyma and cerebral vessels. A large body of genetic, biochemical and in vivo data support a pivotal role for Aβ in the pathological cascade that eventually leads to AD. Patients usually present early symptoms (commonly memory loss) in their sixth or seventh decades of life. The disease progresses with increasing dementia and elevated deposition of Aβ. In parallel, a hyperphosphorylated form of the microtubule-associated protein tau accumulates within neurons, leading to a plethora of deleterious effects on neuronal function. The prevailing working hypothesis regarding the temporal relationship between Aβ and tau pathologies states that Aβ deposition precedes tau aggregation in humans and animal models of the disease. Within this context, it is worth noting that the exact molecular nature of Aβ, mediating this pathological function is presently an issue under intense study. Most likely, there is a continuum of toxic species ranging from lower order Aβ oligomers to supramolecular assemblies such as Aβ fibrils.
The Aβ peptide is an integral fragment of the Type I protein APP (Aβ amyloid precursor protein), a protein ubiquitously expressed in human tissues. Since soluble Aβ can be found in both plasma and cerebrospinal fluid (CSF), and in the medium from cultured cells, APP has to undergo proteolysis. There are three main cleavages of APP that are relevant to the pathobiology of AD, the so-called α-, β-, and γ-cleavages. The α-cleavage, which occurs roughly in the middle of the Aβ domain in APP is executed by the metalloproteases ADAM10 or ADAM17 (the latter also known as TACE). The β-cleavage, occurring at the N terminus of Aβ, is generated by the transmembrane aspartyl protease Beta site APP Cleaving Enzyme1 (BACE1). The γ-cleavage, generating the Aβ C termini and subsequent release of the peptide, is effected by a multi-subunit aspartyl protease named γ-secretase. ADAM10/17 cleavage followed by γ-secretase cleavage results in the release of the soluble p3 peptide, an N-terminally truncated Aβ fragment that fails to form amyloid deposits in humans. This proteolytic route is commonly referred to as the non-amyloidogenic pathway. Consecutive cleavages by BACE1 and γ-secretase generates the intact AP peptide, hence this processing scheme has been termed the amyloidogenic pathway. With this knowledge at hand, it is possible to envision two possible avenues of lowering Aβ production: stimulating non-amyloidogenic processing, or inhibit or modulate amyloidogenic processing. This application focuses on the latter strategy, inhibition or modulation of amyloidogenic processing.
Amyloidogenic plaques and vascular amyloid angiopathy also characterize the brains of patients with Trisomy 21 (Down's Syndrome), Hereditary Cerebral Hemorrhage with Amyloidosis of the Dutch-type (HCHWA-D), and other neurodegenerative disorders. Neurofibrillary tangles also occur in other neurodegenerative disorders including dementia-inducing disorders (Varghese, J., et al, Journal of Medicinal Chemistry, 2003, 46, 4625-4630). β-amyloid deposits are predominately an aggregate of Aβ peptide, which in turn is a product of the proteolysis of amyloid precursor protein (APP). More specifically, Aβ peptide results from the cleavage of APP at the C-terminus by one or more γ-secretases, and at the N-terminus by β-secretase enzyme (BACE), also known as aspartyl protease or Asp2 or Beta site APP Cleaving Enzyme (BACE), as part of the β-amyloidogenic pathway.
BACE activity is correlated directly to the generation of Aβ peptide from APP (Sinha, et al, Nature, 1999, 402, 537-540), and studies increasingly indicate that the inhibition of BACE inhibits the production of Aβ peptide (Roberds, S. L., et al, Human Molecular Genetics, 2001, 10, 1317-1324). BACE is a membrane bound type 1 protein that is synthesized as a partially active proenzyme, and is abundantly expressed in brain tissue. It is thought to represent the major β-secretase activity, and is considered to be the rate-limiting step in the production of amyloid-β-peptide (Aβ).
Drugs that reduce or block BACE activity should therefore reduce Aβ levels and levels of fragments of Aβ in the brain, or elsewhere where Aβ or fragments thereof deposit, and thus slow the formation of amyloid plaques and the progression of AD or other maladies involving deposition of Aβ or fragments thereof. BACE is therefore an important candidate for the development of drugs as a treatment and/or prophylaxis of Aβ-related pathologies such as Down's syndrome, β-amyloid angiopathy such as but not limited to cerebral amyloid angiopathy or hereditary cerebral hemorrhage, disorders associated with cognitive impairment such as but not limited to MCI (“mild cognitive impairment”), Alzheimer's Disease, memory loss, attention deficit symptoms associated with Alzheimer's disease, neurodegeneration associated with diseases such as Alzheimer's disease or dementia including dementia of mixed vascular and degenerative origin, pre-senile dementia, senile dementia and dementia associated with Parkinson's disease, progressive supranuclear palsy or cortical basal degeneration. It would therefore be useful to inhibit the deposition of Aβ and portions thereof by inhibiting BACE through inhibitors such as the compounds provided herein.
The therapeutic potential of inhibiting the deposition of Aβ has motivated many groups to isolate and characterize secretase enzymes and to identify their potential inhibitors.